1. Technical Field of the Invention
The present invention relates to a maximum likelihood estimation apparatus which estimates a maximum likelihood sequence of received signal distorted by multi-path fading in digital mobile communication system, and further estimates adaptively an impulse response of transmission channel.
2. Description of the Prior Art
Transmission quality in the digital mobile communication is degraded by the inter-symbol interference due to the multi-path on transmission channels. The maximum likelihood sequence estimation (MLSE) by Viterbi algorithm is known as one of useful means for estimating the original transmission sequence on the basis of the distorted signal suffering the inter-symbol interference. MLSE has in general a disadvantage that massive signal processing is required, although its equalizing capability is high. The quantity of signal processing is decided by the state number in Viterbi algorithm or the number of taps of MLSE means. The multi-path delay in symbol becomes great, as the transmission rate for multi-media mobile communication becomes high. Therefore, vast calculations are required, if the number of taps is decided by the maximum delay. The number of taps must be suppressed, if quantity of calculations is reduced. However, only a limited part of the impulse response of transmission channel is calculated under the limited calculation capability. Therefore, it is necessary to consider which range of the impulse response is used for the tap coefficients which yield a greater electric power in that range and higher estimation capability.
A conventional technique for selecting optimum range in the impulse response of transmission line is disclosed in JP 11-8575 A (1999) as shown in FIG. 4.
A block diagram of a maximum likelihood sequence estimation receiver as disclosed in JP 11-8575 A (1999) is shown in FIG. 4. An impulse response of a transmission channel is obtained in transmission characteristics detecter 21, when a training signal such as a pseudo-noise (PN) code sequence is received. The impulse response is outputted towards range setting means 22 and maximum likelihood estimation means 23. Range setting means 22 decides a range in the impulse response for setting up the tap coefficients, and outputs timing signal towards maximum likelihood sequence estimation means (MLSE) means 23. MLSE means 23 estimates a maximum likelihood sequence under the Viterbi algorithm by using as many impulses as the number of taps from the head as indicated by the timing signal. Circuit of range setting means 22 is made small by employing amplitude calculation in place of power calculation. Concretely, absolute value calculation means 24 obtains amplitudes of impulse response, then, accumulator 25 accumulates as many amplitudes as the number of taps, and further, obtains the accumulation value for one period of the training signal, by shifting the impulse response. Maximum value detecting means 26 generates a timing signal by detecting a position where the accumulation value becomes maximum.
However, it becomes difficult to estimate correctly the maximum likelihood sequence, when the impulse response obtained by using the training signal becomes different greatly from the actual transmission characteristics as time goes by. Therefore, adaptive algorithm such as least mean square (LMS) and recursive least squares (RLS) are employed, when the transmission characteristics change greatly, depending on time. However, even when the adaptive estimation is employed, the selected range of the impulses for setting up the tap coefficients remains unchanged. Therefore, the estimation of maximum likelihood sequence becomes incorrect, when the optimum range for determining the impulse response changes.
An object of the present invention is to renew adaptively the impulse response as well as to renew adaptively the optimum range of the impulse response, in order to estimate correctly the maximum likelihood sequence.
The maximum likelihood sequence estimation apparatus of the present invention decides an optimum range of the impulse response, every time when the impulse response is estimated adaptively, and further decides received signal used for the adaptive estimation of the impulse response and the maximum likelihood sequence estimation.
According to the present invention, MLSE is executed by using the impulses at the optimum range, even when the optimum range of the impulses is changed in time. Therefore, the MLSE apparatus of the present invention estimates correctly the maximum likelihood sequence.